the relative path length of thedirect solar beam radiancethrough the atmosphere. When the sun is directly above a sea-levellocation the path length is defined as airmass 1 (AM 1.0). AM 1.0 is notsynonymous with solar noon because the sun is usually not directlyoverhead at solar noon in most seasons and locations. When the angle ofthe sun from zenith (directly overhead) increases,the airmass increasesapproximately by the secant of the zenith angle. A better calculation(Kasten, F. and A. T. Young(1989). Revised optical air mass tables and approximation formula.Applied Optics 28 (22), 4735-4738 )  follows:
m = 1.0 / [ cos(Z) + 0.50572 * (96.07995 – Z)-1.6364]
where Z is the solar zenith angle.
The figure below illustrates the concept of airmass.